Gallium nitride based diodes with low forward voltage and low reverse current operation

ABSTRACT

New Group III based diodes are disclosed having a low on state voltage (V f ) and structures to keep reverse current (I rev ) relatively low. One embodiment of the invention is Schottky barrier diode made from the GaN material system in which the Fermi level (or surface potential) of is not pinned. The barrier potential at the metal-to-semiconductor junction varies depending on the type of metal used and using particular metals lowers the diode&#39;s Schottky barrier potential and results in a V f  in the range of 0.1-0.3V. In another embodiment a trench structure is formed on the Schottky diodes semiconductor material to reduce reverse leakage current. and comprises a number of parallel, equally spaced trenches with mesa regions between adjacent trenches. A third embodiment of the invention provides a GaN tunnel diode with a low V f  resulting from the tunneling of electrons through the barrier potential, instead of over it. This embodiment can also have a trench structure to reduce reverse leakage current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to diodes, and more particularly to galliumnitride (GaN) based diodes exhibiting improved forward voltage andreverse leakage current characteristics.

2. Description of the Related Art

Diode rectifiers are one of the most widely used devices for low voltageswitching, power supplies, power converters and related applications.For efficient operation it is desirable for diodes to have low on-statevoltage (0.1-0.2V or lower), low reverse leakage current, high voltageblocking capability (20-30V), and high switching speed.

The most common diodes are pn-junction diodes made from silicon (Si)with impurity elements introduced to modify, in a controlled manner, thediode's operating characteristics. Diodes can also be formed from othersemiconductor materials such as Gallium Arsenide (GaAs) and siliconcarbide (SiC). One disadvantage of junction diodes is that duringforward conduction the power loss in the diode can become excessive forlarge current flow.

Schottky barrier diodes are a special form of diode rectifier thatconsist of a rectifying metal-to-semiconductor barrier area instead of apn junction. When the metal contacts the semiconductor a barrier regionis developed at the junction between the two. When properly fabricatedthe barrier region will minimize charge storage effects and improve thediode switching by shortening the turn-off time. [L. P. Hunter, Physicsof Semiconductor Materials, Devices, and Circuits, SemiconductorDevices, Page 1-10 (1970)] Common Schottky diodes have a lower turn-onvoltage (approximately 0.5V) than pn-junction diodes and are moredesirable in applications where the energy losses in the diodes can havea significant system impact (such as output rectifiers in switchingpower supplies).

One way to reduce the on-state voltage below 0.5V in conventionalSchottky diodes is to reduce their surface barrier potential. This,however, results in a trade-off of increased reverse leakage current. Inaddition, the reduced barrier can degrade high temperature operation andresult in soft breakdown characteristics under reverse bias operation.

Also, Schottky diodes are commonly made of GaAs and one disadvantage ofthis material is that the Fermi level (or surface potential) is fixed orpinned at approximately 0.7 volts. As a result, the on-state forwardvoltage (V_(f)) is fixed. Regardless of the type of metal used tocontact the semiconductor, the surface potential cannot be lowered tolower the V_(f).

More recently, silicon based Schottky rectifier diodes have beendeveloped with a somewhat lower V_(f). [IXYS Corporation, Si Based PowerSchottky Rectifier, Part Number DSS 20-0015B; International Rectifier,Si Based Shottky Rectifier, Part Number 11DQ09]. The Shottky barriersurface potential of these devices is approximately 0.4V with the lowerlimit of V_(f) being approximately 0.3-0.4 volts. For practical purposesthe lowest achievable Shottky barrier potential is around 0.4 volts withregular metalization using titanium. This results in a V_(f) ofapproximately 0.25V with a current density of 100 A/cm².

Other hybrid structures have been reported with a V_(f) of approximately0.25V (with a barrier height of 0.58V) with operating current density of100 A/cm². [M. Mehrotra, B. J. Baliga, “The Trench MOS Barrier Shottky(TMBS) Rectifier”, International Electron Device Meeting, 1993]. Onesuch design is the junction barrier controlled Schottky rectifier havinga pn-junction used to tailor the electric fields to minimize reverseleakage. Another device is the trench MOS barrier rectifier in which atrench and a MOS barrier action are used to tailor the electrical fieldprofiles. One disadvantage of this device is the introduction of acapacitance by the pn-junction. Also, pn-junctions are somewhatdifficult to fabricate in Group III nitride based devices.

The Gallium nitride (GaN) material system has been used inopto-electronic devices such as high efficiency blue and green LEDs andlasers, and electronic devices such as high power microwave transistors.GaN has a 3.4 eV wide direct bandgap, high electron velocity (2×10⁷cm/s), high breakdown fields (2×10⁶ V/cm) and the availability ofheterostructures.

SUMMARY OF THE INVENTION

The present invention provides new Group III nitride based diodes havinga low V_(f). Embodiments of the new diode also include structures tokeep reverse current (I_(rev)) relatively low.

The new diode is preferably formed of the GaN material system, andunlike conventional diodes made from materials such as GaAs, the Fermilevel (or surface potential) of GaN is not pinned at its surface states.In GaN Schottky diodes the barrier height at the metal-to-semiconductorjunction varies depending on the type of metal used. Using particularmetals will lower the diode's Schottky barrier height and result in aV_(f) in the range of 0.1-0.3V.

The new GaN Schottky diode generally includes an n+ GaN layer on asubstrate, and an n− GaN layer on the n+ GaN layer opposite thesubstrate. Ohmic metal contacts are included on the n+ GaN layer,isolated from the n− GaN layer, and a Schottky metal layer is includedon the n− GaN layer. The signal to be rectified is applied to the diodeacross the Schottky metal and ohmic metal contacts. When the Schottkymetal is deposited on the n− GaN layer, a barrier potential forms at thesurface of said n− GaN between the two. The Schottky metal layer has awork function, which determines the height of the barrier potential.

Using a metal that reduces the Schottky barrier potential results in alow V_(f), but can also result in an undesirable increase in I_(rev). Asecond embodiment of the present invention reduces I_(rev) by includinga trench structure on the diode's surface. This structure prevents anincrease in the electric field when the new diode is under reverse bias.As a result, the Schottky barrier potential is lowered, which helpsreduce I_(rev).

The trench structure is preferably formed on the n− GaN layer, andcomprises a number of parallel, equally spaced trenches with mesaregions between adjacent trenches. Each trench has an insulating layeron its sidewalls and bottom surface. A continuous Schottky metal layeris on the trench structure, covering the insulating layer and the mesasbetween the trenches. Alternatively, the sidewalls and bottom surface ofeach trench can be covered with metal instead of an insulator, with themetal electrically isolated from the Schottky metal. The mesa regionshave a doping concentration and width chosen to produce the desiredredistribution of electrical field under the metal-semiconductorcontact.

A third embodiment of the invention provides a GaN tunnel diode with alow V_(f) resulting from the tunneling of electrons through the barrierpotential, instead of over it. This embodiment has a substrate with ann+ GaN layer sandwiched between the substrate and an n− GaN layer. AnAlGaN barrier layer is included on the n− GaN layer opposite the n+ GaNlayer. An Ohmic contact is included on the n+ GaN layer and a topcontact is included on the AlGaN layer. The signal to be rectified isapplied across the Ohmic and top contacts.

The barrier layer design maximizes the forward tunneling probabilitywhile the different thickness and Al mole fraction of the barrier layerresult in different forward and reverse operating characteristics. At aparticular thickness and Al mole fraction, the diode has a low V_(f) andlow I_(rev). Using a thicker barrier layer and/or increasing the Al moleconcentration decreases V_(f) and increases I_(rev). As the thickness ormole fraction is increased further, the new diode will assume ohmicoperating characteristics, or become a conventional Schottky diode.

These and other further features and advantages of the invention wouldbe apparent to those skilled in the art from the following detaileddescription, taking together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a GaN Schottky diode embodiment of theinvention;

FIG. 2 is a diagram showing the work function of common metals versestheir atomic number;

FIG. 3 is a band diagram for the diode shown in FIG. 1;

FIG. 4 is a sectional view of another embodiment of the GaN Schottydiode of FIG. 1, having a trench structure to reduce reverse currentleakage;

FIG. 5 is a sectional view of a tunnel diode embodiment of theinvention;

FIG. 6 is a band diagram for the tunnel diode of FIG. 5 having a barrierlayer with a thickness of 22 Å and 30% Al mole fraction;

FIG. 7 is a diagram showing the voltage/current characteristics of thenew tunnel diode having the band diagram of FIG. 6;

FIG. 8 is a band diagram for the tunnel diode of FIG. 5 having a barrierlayer with a thickness of 30 Å and 30% Al mole fraction;

FIG. 9 is a diagram showing the voltage/current characteristics of thenew tunnel diode having the band diagram of FIG. 8;

FIG. 10 is a band diagram for the tunnel diode of FIG. 5 having abarrier layer with a thickness of 38 Å and 30% Al mole fraction;

FIG. 11 is a diagram showing the voltage/current characteristics of thenew tunnel diode having the band diagram of FIG. 10; and

FIG. 12 is a sectional view of a tunnel diode embodiment of theinvention having a trench structure to reduce reverse current leakage.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a Schottky diode 10 constructed in accordance with thepresent invention having a reduced metal-to-semiconductor barrierpotential. The new diode is formed of the Group III nitride basedmaterial system or other material systems where the Fermi level is notpinned at its surface states. Group III nitrides refer to thosesemiconductor compounds formed between nitrogen and the elements inGroup III of the periodic table, usually aluminum (Al), gallium (Ga),and indium (In). The term also refers to ternary and tertiary compoundssuch as AlGaN and AlInGaN. The preferred materials for the new diode areGaN and AlGaN.

The new diode 10 comprises a substrate 11 that can be either sapphire(Al₂O₃), silicon (Si) or silicon carbide (SiC), with the preferredsubstrate being a 4H polytype of silicon carbide. Other silicon carbidepolytypes can also be used including 3C, 6H and 15R polytypes. AnAl_(x)Ga_(1-x)N buffer layer 12 (where x in between 0 and 1) is includedon the substrate 11 and provides an appropriate crystal structuretransition between the silicon carbide substrate and the remainder ofthe diode 10.

Silicon carbide has a much closer crystal lattice match to Group IIInitrides than sapphire and results in Group III nitride films of higherquality. Silicon carbide also has a very high thermal conductivity sothat the total output power of Group III nitride devices on siliconcarbide is not limited by the thermal dissipation of the substrate (asis the case with some devices formed on sapphire). Also, theavailability of silicon carbide substrates provides the capacity fordevice isolation and reduced parasitic capacitance that make commercialdevices possible. SiC substrates are available from Cree Research, Inc.,of Durham, N.C. and methods for producing them are set forth in thescientific literature as well as in a U.S. Pat. Nos. Re. 34,861;4,946,547; and 5,200,022.

The new diode 10 has an n+ GaN layer 12 on a substrate 11 and an n−layer of GaN 13 on the n+ GaN layer 12, opposite the substrate 11. Then+ layer 12 is highly doped with impurities to a concentration of atleast 10¹⁸ per centimeter cubed (cm³), with the preferable concentrationbeing 5 to 10 times this amount. The n− layer 13 has a lower dopingconcentration but is still n− type and it preferably has an impurityconcentration in the range of 5×10¹⁴ to 5×10¹⁷ per cm³. The n-layer 13is preferably 0.5-1 micron thick and the n+ layer 12 is 0.1 to 1.5microns thick, although other thicknesses will also work.

Portions of the n− GaN layer 13 are etched down to the n+ layer andohmic metal contacts 14a and 14b are included on the n+ GaN layer in theetched areas so that they are electrically isolated from the n− GaNlayer 13. In an alternative embodiment, one or more ohmic contacts canbe included on the surface of the substrate that is not covered by then+ GaN layer 12. This embodiment is particularly applicable tosubstrates that are n-type. A Schottky metal layer 16 is included on then− GaN layer 13, opposite the n+ GaN layer 12.

The work function of a metal is the energy needed to take an electronout of the metal in a vacuum and the Fermi level of a material is theenergy level at which there is a 50% probability of finding a chargedcarrier. A semiconductor's electron affinity is the difference betweenits vacuum energy level and the conduction band energy level.

As described above, the surface Fermi level of GaN is unpinned and as aresult, Schottky metals with different work functions result indifferent barrier potentials. The barrier potential is approximated bythe equation:Barrier Height=work function−the semiconductor's electron affinityFIG. 2 is a graph 20 showing the metal work function 21 for variousmetal surfaces in a vacuum, verses the particular metal's atomic number22. The metal should be chosen to provide a low Schottky barrierpotential and low V_(f), but high enough so that the reverse currentremains low. For example, if a metal were chosen having a work functionequal to the semiconductor's electron affinity, the barrier potentialapproaches zero. This results in a V_(f) that approaches zero and alsoincreases the diode's reverse current such that the diode becomes ohmicin nature and provides no rectification.

Many different metals can be used to achieve a low barrier height, withthe preferred metals including Ti(4.6 work function) 23, Cr(4.7) 24,Nb(4.3) 25, Sn(4.4) 26, W(4.6) 27 and Ta (4.3) 28. Cr 24 results in anacceptable barrier potential and is easy to deposit by conventionalmethods.

FIG. 3 shows a typical band diagram 30 for the new Schottky barrierdiode taken on a vertical line through the diode. It shows the energylevels of Schottky metal 31, the GaN semiconductor layers 32, and theShottky barrier potential 33.

Prior to contact of the GaN semiconductor material by the Schottkymetal, the Fermi energy levels of the two are not the same. Once thecontact is made and the two materials become a single thermodynamicsystem, a single Fermi level for the system results. This isaccomplished by the flow of electrons from the semiconductor material,which has a higher Fermi level, to the Schottky metal, which has a lowerFermi level. The electrons of the semiconductor lower their energy byflowing into the metal. This leaves the ionized donor levels of thesemiconductor somewhat in excess of the number of its free electrons andthe semiconductor will have a net positive charge. Electrons that haveflowed from the semiconductor into the metal cause the metal have anegative electrostatic charge. The energy levels of the semiconductorare accordingly depressed, and those of the metal are raised. Thepresence of this surface charge of electrons and the presence ofunneutralized charge ionized donor levels of the semiconductor createthe dipole layer which forms the barrier potential.

In operation, the signal to be rectified by the new Schottky diode 10 isapplied across the Schottky metal 14 and the ohmic contacts 14 a and 14b. The rectification of the signal results from the presence of thebarrier potential at the surface of the n− GaN layer 13, which inhibitsthe flow of charged particles within the semiconductor. When theSchottky metal 16 is positive with respect to the semiconductor (forwardbias), the energy at the semiconductor side of the barrier is raised. Alarger number of free electrons on the conduction band are then able toflow into the metal. The higher the semiconductor side is raised, themore electrons there are at an energy above the top of the barrier,until finally, with large bias voltages the entire distribution of freeelectrons in the semiconductor is able to surmount the barrier. Thevoltage verses current characteristics become Ohmic in nature. The lowerthe barrier the lower the V_(f) necessary to surmount the barrier.

However, as discussed above, lowering the barrier level can alsoincrease the reverse leakage current. When the semiconductor is madepositive with respect to the metal (reverse bias), the semiconductorside of the barrier is lowered relative to the metal side so that theelectrons are free to flow over the top of the barrier to thesemiconductor unopposed. The number of electrons present in the metalabove the top of the barrier is generally very small compared to thetotal number of electrons in the semiconductor. The result is a very lowcurrent characteristic. When the voltage is large enough to cut-off allflow of electrons, the current will saturate. The lower the barrierpotential, the smaller reverse biases needed for the current tosaturate.

FIG. 4 shows another embodiment of the new GaN Schottky diode 40 thataddresses the problem of increased reverse current with decreasedbarrier height. The diode 40 is similar to the above embodiment having asimilar substrate 41, n+ GaN layer 42, and Ohmic metal contacts 43 a and43 b, that can alternatively be included on the surface of thesubstrate. It also has an n− GaN layer 44, but instead of this layerbeing planar, it has a two dimensional trench structure 45 that includestrenches 46 in the n−GaN layer. The preferred trench structure 45includes trenches 46 that are parallel and equally spaced with mesaregions 49 remaining between adjacent trenches. Each trench 46 has aninsulating layer 47 covering its sidewalls 46 a and bottom surface 46 b.Many different insulating materials can be used with the preferredmaterial being silicon nitride (SiN). A Schottky metal layer 48 isincluded over the entire trench structure 45, sandwiching the insulatinglayer between the Schottky metal and the trench sidewalls and bottomsurface, and covering the mesa regions 49. The mesa regions provide thedirect contact area between the Schottky metal and the n− GaN layer 44.Alternatively, each trench can be covered by a metal instead of aninsulator. In this embodiment, the Schottky metal should be insulatedand/or separated from the trench metal.

The mesa region 49 has a doping concentration and width chosen toproduce a redistribution of electrical field under the mesa'smetal-semiconductor junction. This results in the peak of the diodeselectrical field being pushed away from the Schottky barrier and reducedin magnitude. This reduces the barrier lowering with increased reversebias voltage, which helps prevent reverse leakage current fromincreasing rapidly.

This redistribution occurs due to the coupling of the charge in the mesa49 with the Schottky metal 48 on the top surface and with the metal onthe trench sidewalls 46 a and bottom surface 46 b. The depletion thenextends from both the top surface (as in a conventional Schottkyrectifier) and the trench sidewalls 46 a, depleting the conduction areafrom the sidewalls. The sidewall depletion reduces the electrical fieldunder the Schottky metal layer 48 and can also be thought of as“pinching off” the reverse leakage current. The trench structure 45keeps the reverse leakage current relatively low, even with a lowbarrier potentials and a low V_(f).

The preferred trench structure 45 has trenches 46 that are one to twotimes the width of the Schottky barrier area. Accordingly, if thebarrier area is 0.7 to 1.0 microns, the trench width could be in therange of 0.7 to 2 microns.

The above diodes 10 and 40 are fabricated using known techniques. Theirn+ and n− GaN layers are deposited on the substrate by known depositiontechniques including but not limited to metal-organic chemical vapordeposition (MOCVD). For diode 10, the n− GaN layer 13 is etched to then+ GaN layer 12 by known etching techniques such as chemical, reactiveion etching (RIE), or ion mill etching. The Schottky and Ohmic metallayers 14, 14 b and 16 are formed on the diode 10 by standardmetallization techniques.

For diode 40, after the n+ and n− layers 42 and 44 are deposited on thesubstrate, the n− GaN layer 44 is etched by chemical or ion mill etchingto form the trenches 46. The n− GaN layer 44 is further etched to the n+GaN layer 42 for the ohmic metal 43 a and 43 b. The SiN insulation layer47 is then deposited over the entire trench structure 45 and the SiNlayer is etched off the mesas 49. As a final step, a continuous Schottkymetal layer 48 is formed by standard metalization techniques over thetrench structure 45, covering the insulation layers 47 and the exposedtrench mesas 49. The ohmic metal is also formed on the n+ GaN layer 42by standard metalization techniques. In the embodiments of the trenchdiode where the trenches are covered by a metal, the metal can also bedeposited by standard metalization techniques.

Tunnel Diode

FIG. 5 shows another embodiment 50 of the new diode wherein V_(f) is lowas a result of electron tunneling through the barrier region underforward bias. By tunneling through the barrier electrons do not need tocross the barrier by conventional thermionic emission over the barrier.

Like the embodiments in FIGS. 1 and 4, the new tunnel diode 50 is formedfrom the Group III nitride based material system and is preferablyformed of GaN, AlGaN or InGaN, however other material systems will alsowork. Combinations of polar and non-polar materials can be usedincluding polar on polar and polar on non-polar materials. Some examplesof these materials include complex polar oxides such as strontiumtitanate, lithium niobate, lead zirconium titanate, andnon-complex/binary oxides such as zinc oxide. The materials can be usedon silicon or any silicon/dielectric stack as long as tunneling currentsare allowed.

The diode 50 has a substrate 51 comprised of either sapphire, siliconcarbide (SiC) or silicon Si, with SiC being the preferred substratematerial for the reasons outlined above. The substrate has an n+ GaNlayer 52 on it, with an n− GaN layer 53 on the n+ GaN layer 52 oppositethe substrate 51. An AlGaN barrier layer 54 is included on the n− GaNlayer opposite the n+ GaN template layer 52. At the edges of the diode50, the barrier layer 54 and n− GaN layer 53 are etched down to the n+GaN layer 52 and ohmic metal contacts 55 a and 55 b are included on thelayer 52 in the etched areas. As with the above structures, the ohmiccontacts can also be included on the surface of the substrate. A metalcontact layer 56 is included on the AlGaN barrier layer 54, opposite then−GaN layer 53. The signal to be rectified is applied across the ohmiccontacts 55 a and 55 b and top metal contact 56.

The AlGaN barrier layer 54 serves as a tunnel barrier. Tunneling acrossbarriers is a quantum mechanical phenomenon and both the thickness andthe Al mole fraction of the layer 54 can be varied to maximize theforward tunneling probability. The AlGaN-GaN material system a has builtin piezoelectric stress, which results in piezoelectric dipoles.Generally both the piezoelectric stress and the induced charge increaseswith the barrier layer thickness. In the forward bias, the electronsfrom the piezoelectric charge enhance tunneling since they are availablefor conduction so that the number of states from which tunneling canoccur is increased. Accordingly the new tunnel diode can be made ofother polar material exhibiting this type of piezoelectric charge.

However, under a reverse bias the piezoelectric charge also allows anincrease in the reverse leakage current. The thicker the barrier layeror increased Al mole fraction, results in a lower V_(f) but also resultsin an increased I_(rev). Accordingly, there is an optimum barrier layerthickness for a particular Al mole fraction of the barrier layer toachieve operating characteristics of low V_(f) and relatively lowI_(rev).

FIGS. 6-11 illustrate the new diode's rectification characteristics forthree different thicknesses of an AlGaN barrier layer with 30% Al. Foreach thickness there is a band energy diagram and a correspondingvoltage vs. current graph.

FIG. 6 shows the band diagram 60 for the tunnel diode 50 having 22Athick barrier layer 54. It shows a typical barrier potential 61 at thejunction between the barrier layer 63 and the n− GaN semiconductor layer62. The top contact metal 64 is on the barrier layer 63, opposite thesemiconductor layer. FIG. 7 shows a graph 70 plotting the correspondingcurrent vs. voltage characteristics of the diode in FIG. 6. It has aV_(f) 71 of approximately 0.1V and low reverse current (I_(rev)) 72.

FIG. 8 shows a band diagram 80 for the same tunnel diode with a 30 Åthick barrier layer. The increase in the barrier layer thicknessincreases the barrier region's piezoelectric charge, thereby enhancingtunneling across the barrier. This flattens the barrier potential 81 atthe junction between the barrier layer 82 and the n−GaN layer 83.Charges do not need to overcome the barrier when a forward bias isapplied, greatly reducing the diode's V_(f). However, the flattenedbarrier also allows for increase reverse leakage current (I_(rev)). FIG.9 is a graph 90 showing the V_(f) 91 that is lower than the V_(f) inFIG. 7. Also, I_(rev) 92 is increased compared to I_(rev) in FIG. 7.

FIG. 10 shows a band diagram 100 for the same tunnel diode with a 38 Åthick barrier layer. Again, the increase in the barrier layer thicknessincreases the piezoelectric charge. At this thickness, the barrierpotential 101 between the barrier layer 102 and n− GaN layer tails downnear the junction between the barrier layer and n− GaN layer, whichresults in there being no barrier to charges in both forward and reversebias. FIG. 11 shows a graph 110 of the corresponding current vs. voltagecharacteristics. The diode 100 experiences immediate forward and reversecurrent in response to forward and reverse bias such that the diodebecomes ohmic in nature.

In the case where the mole concentration of aluminum in the barrierlayer is different, the thicknesses of the layers would be different toachieve the characteristics shown in FIGS. 6 through 11.

FIG. 12 shows the new tunneling diode 120 with a trench structure 121 toreduce reverse current. Like the Schottky diode 40 above, the trenchstructure includes a number of parallel, equally spaced trenches 122,but in this diode, they are etched through the AlGaN barrier layer 123and the n− GaN layer 124, to the n+ GaN layer 125 (AP GaN Template).There are mesa regions 126 between adjacent trenches 122. The trenchsidewalls and bottom surface have an insulation layer 127 with the topSchottky metal layer 128 covering the entire trench structure 121. Thetrench structure functions in the same way as the embodiment above,reducing the reverse current. This is useful for the tunnel diodeshaving barrier layers of a thickness that results in immediate forwardcurrent in response to forward voltage. By using trench structures, thediode could also have improved reverse current leakage. Also like above,the trench sidewalls and bottom surface can be covered by a metal aslong as it is isolated from the Schottky metal layer 128. Although thepresent invention has been described in considerable detail withreference to certain preferred configurations thereof, other versionsare possible. Therefore, the spirit and scope of the appended claimsshould not be limited to the preferred versions described in thespecification.

1-51. (canceled)
 52. A tunneling diode comprising: an n+ doped layer; ann− doped layer adjacent to said n+ doped layer; a barrier layer adjacentto said n− doped layer, opposite said n+ layer; and a metal layer onsaid barrier layer, opposite said n-doped layer, said n+ doped, n− dopedand barrier layers made from a material system having a piezoelectricstress, said piezoelectric stress causing said diode's on-statethreshold voltage to be low as a result of enhanced electron tunnelingthrough the potential barrier under forward bias.
 53. The diode of claim52, wherein said piezoelectric stress in said barrier layer providespiezoelectric dipoles having electrons available for conduction to lowerthe diode's on-state threshold voltage by enhancing electron tunneling.54. The diode of claim 52, wherein the number of said piezoelectricdipoles increases as the thickness of said barrier layer increases,while still allowing tunneling currents.
 55. The diode of claim 52,further comprising a substrate adjacent to said n+ doped layer oppositesaid n− doped layer, said substrate comprising sapphire, silicon carbideor silicon.
 56. The diode of claim 52, wherein said n+ doped layer, n−doped layer and barrier layer comprise polar materials.
 57. The diode ofclaim 52, wherein said n+ doped layer, n− doped layer and barrier layerare from the Group III nitride material system.
 58. The diode of claim52, wherein said n+ doped layer is GaN, said n− doped layer is GaN, andsaid barrier layer is AlGaN.
 59. The diode of claim 52, wherein said n+doped layer, n-doped layer and barrier layer are formed from polar ornon-polar materials, or combinations thereof.
 60. The diode of claim 52,wherein said n+ doped layer, n-doped layer and barrier layer are formedfrom complex polar oxides such as strontium titanate, lithium niobate,lead zirconium titanate, or combinations thereof.
 61. The diode of claim52, wherein said n+ doped layer, n-doped layer and barrier layer frombinary polar oxides such as zinc oxide.
 62. The diode of claim 52,further comprising a trench structure in said barrier and n− dopedlayers, said diode experiencing a reverse leakage current under reversebias, said trench structure reducing the amount of said reverse leakagecurrent.
 63. The diode of claim 62, wherein said trench structurecomprises a plurality of trenches in said barrier and said n− layershaving mesa regions between adjacent trenches, each of said trencheshaving opposing sidewalls and a bottom surface, said sidewalls andbottom surface of each of said trenches being coated by a layer ofinsulating material, said metal layer covering each of said trench'slayer of insulating material and said mesa regions so that each saidlayer of insulating material is sandwiched between said metal layer andits respective said sidewalls and bottom surface.
 64. The diode of claim63, wherein a portion of said insulating material is replaced by a metalwith a high work function, said metal with a high work function beingseparated from said metal layer by said insulating material.
 65. Thediode of claim 52, further comprising an ohmic contact on said n+ dopedlayer.
 66. A tunneling diode, comprising: an n+ doped nitride layer; ann− doped nitride layer adjacent to said n+ doped nitride layer; abarrier layer adjacent to said n− doped nitride layer, opposite said n+nitride layer; and a metal layer on said barrier layer, opposite said n−doped nitride layer, said n− doped nitride layer forming a junction withsaid barrier layer, said junction having a potential barrier, saidbarrier layer having a spontaneous and piezoelectric polarization thatresults in dipoles having electrons available for conduction whichcauses said diode's on-state threshold voltage to be low as a result ofelectron tunneling through the potential barrier under forward bias. 67.A tunneling diode, comprising: an n+ doped layer; an n− doped layeradjacent to said n+ doped layer; a barrier layer adjacent to said n−doped layer, opposite said n+ layer; a trench structure in said barrierand n− doped layers; and a metal layer on said barrier layer and trenchstructure, opposite said n− doped layer, said n− doped layer, forming ajunction with said barrier layer that has a barrier potential, saidjunction having a potential barrier, said barrier layer having aspontaneous and piezoelectric polarization that results in dipoleshaving electrons available for conduction said barrier layer having athickness chosen to obtain a desired on-state threshold voltage as aresult of electron tunneling through the potential barrier under forwardbias, said trench structure reducing the amount of reverse leakagecurrent under reverse bias.